Circle Area Calculator — πr² | How to use and interpret results
Calculate the area and circumference of a circle from its radius. The two outputs help separate surface coverage from the length around an edge.
- Runs in your browser
- Nothing is uploaded
- No signup; device limits apply
Numbers stay on your device. Nothing is sent to a server.
How to use this tool
Enter a nonnegative radius. If your measurement is a diameter, divide it by two first and keep one length unit throughout the calculation.
Results and examples
A radius of 5 produces an area of about 78.539816 and a circumference of about 31.415927. For a radius in centimetres, these mean square centimetres and centimetres respectively.
Worked example
A radius of 5 produces an area of about 78.539816 and a circumference of about 31.415927. For a radius in centimetres, these mean square centimetres and centimetres respectively.
Limitations
- This is not an ellipse or sphere calculator.
- Material waste, fitting allowances and unit conversion are outside the formula.
Fields
| Field | Guidance and constraints |
|---|---|
| Radius | Accepted range: 0–1000000000000 · Input step: 0.0001 |
Data handling
Processing stays in the current browser tab; the site does not upload tool input for the calculation itself.
Sources
Frequently asked questions
What should I enter for a diameter of 10?
Enter a radius of 5.
Is zero accepted?
Yes. A zero radius produces zero area and zero circumference.
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